There are many systems for the realization of a wireless local area networks (WLAN). From among the large set of protocols described in 802.16 and 802.16e is a subset known as WiMAX, as described in WiMAX Forum “Mobile WiMAX—Part I: Technical Overview and Performance Evaluation” published August 2006, which is incorporated in its entirety by reference. FIG. 1A shows a WiMAX base station C1 102 which operates within the boundaries of a cell 110. In one prior art embodiment, the cell is divided into a plurality of segments, shown as segment 0 104, segment 1 106, and segment 2 108, and the base station uses an adaptive antenna array to generate a preamble which provides preferential signal strength in each segment 104, 106, 108, in succession. One such transmit radiation pattern for an arbitrary cell segment such as segment 2 is shown as 111.
FIG. 1B shows a prior art network of WiMAX cells such as the one described for FIG. 1A, where each cell base station C1, C2, C3, C4, C5, C6, C7 is synchronized to an external time clock such as a GPS timebase, and all base stations operate together to simultaneously transmit a series of WiMAX frames as will be discussed for FIG. 2A. A mobile station M1 120 is located within a cell and is separated from possible base stations C1 through C7 by distances unknown to the mobile station 120. For example, mobile station M1 120 may simultaneously receive at least the preamble part of WiMAX frames from each base station, such that M1 120 receives from C1 102 segment 1 (S1) on path 122, C2 124 segment 2 (S2) on path 126, and B3 128 segment 0 on path 130.
FIG. 2A shows a WiMAX frame 200, which includes a preamble part 202, a header part 204, a downlink data part 206, and an optional uplink data part 208. Each base station B1, B2, etc transmits these 5 ms frames 200 simultaneously using an external reference for timing synchronization, such as GPS timing reference, so that station M1 120 receives the frames 200 from all surrounding stations substantially simultaneously, other than time of flight delays and timing resolution errors. The frames 200 are modulated with Orthogonal Frequency Division Multiplexing Multiple Access (OFDMA) subcarrier tones, whereby a communications channel having frequency bandwidth is subdivided into as many as 2048 subcarriers, each subcarrier separated by 10.94 Khz and combinations of these subcarriers are selected such that adjacent base stations minimize the simultaneous use of a particular subcarrier, such that a station M1 120 is able to simultaneously receive frames 200 from a plurality of different base stations such as B1, B2, B3, each of which station may have more than one spatial segment, such that the mobile station M1 may use the preamble part of the frame to discern which base stations are nearby, which segment of a particular base station is being received, and which one of the possible base stations from nearby cells is the best candidate for selection for the mobile station M1 to attach and form a wireless LAN.
All of the cell stations C1 through C7 are sending a unique preamble on each segment, and FIG. 2B shows the preambles of a WiMAX frame 202 being simultaneously transmitted by 3 such cell station and segment combinations. Preamble 221 shows cell 3 segment 0 (C3S0), preamble 223 shows cell 1 segment 1 (C1S1), and preamble 225 shows cell 2 segment 2 (C2S2), corresponding to the preambles received by mobile station M1 120 from cell 3 path 130, cell 2 path 126, and cell 1 path 122. The objective of the mobile station M1 120 which is simultaneously receiving the superset of unique and non-interfering subcarrier combinations from all surrounding stations such as 222, 224, 226, is to identify the strongest received unique preamble from one of these stations and select the associated station for association into the network.
FIG. 2C shows a subset of subcarrier frequencies versus time, including a particular simplified exemplar preamble for cell 3 segment 0 C3S0 246. In this simplified example, C3S0 uses subcarriers 56, 59, 62, 65, 68, 71, of which subcarriers 56, 68, and 71 are active. Cells on segment 1 would use subcarriers 57, 60, 63, 66, 69, 72, unique combinations of which would be active, and cells on segment 2 would use subcarriers 58, 61, 64, 67, 70, 73. In this manner, each segment is operating on a particular subset of ⅓ subcarriers which is separable from the others, as will be described later.
FIG. 3 shows a time domain diagram for the preambles received by station M1 120, which include the unique subcarrier combinations for the preambles assigned to each cell and defined by the WiMAX standards for the present example C1S1, C2S2, and S3S0. The received combinations of subcarriers have an offset frequency which is related to the frequency offset from transmitter to receiver. For example, a transmitter which is operating at 2.4 Ghz may have an allowable frequency variation of ±20 ppm, corresponding to ±50 Khz of variation at the transmitter, which has the effect of linearly shifting the entire group of transmitted subcarriers by ±50 Khz. The base stations synchronize in carrier frequency to each other such that surrounding base stations have subcarrier frequency offsets relative to each other which are very small. Since the subcarrier spacing for OFDMA is 10.94 Khz, the receiver subcarriers may be offset from the originally transmitted subcarriers, and the uncertainty of the receiver's ability to locate particular subcarriers can vary by ±5 subcarriers. Typically, a receiver will perform “fractional offset” during symbol timing, whereby the received subcarriers will be placed with the peaks at the sample windows of the FFT. This is shown in FIG. 4A, where a group of subcarriers 404 is received as a superposition of subcarriers from a plurality of stations such as C1S1, C2S2, C3S0 and others. The combination of subcarriers for preamble C3S0 is shown shaded as 420. The first step of the receiver is to remove any frequency ramps, thereby producing a set of subcarriers with a fixed frequency offset 402 of FIG. 4A, with the objective of placing a subcarrier peak 403 at the center of a subcarrier FFT sample point such as 421 of FIG. 4B which shows the removal of fractional offset.
FIG. 4B also shows the remaining integer offset, such that of a particular preamble subcarrier combination such as that of cell/segment 420 corresponding to C3S0 having the subcarrier pattern [1,−1,−1,−1,1,1] which is present with an integer offset of −2. The objective of cell station selection is to search for the matching subcarrier pattern such as [1,−1,−1,−1,1,1], which is done from the complete subcarrier set [(1,x,y)(0,x,y)(0,x,y)(0,x,y)(1,x,y)(1,x,y)] where x and y represent subcarriers which can be ignored for segments 1 and 2, respectively.